1. Field of the Invention
This invention relates to the fabrication of ink jet printheads and more particularly to the method of bonding multiple part printheads together without the adhesive obstructing the flow of ink.
2. Description of the Prior Art
Drop-on-demand ink jet printing systems can be divided into two basic types. One type uses a a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle, and the other type uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. This latter type is referred to as thermal ink jet printing or bubble ink jet printing. Generally, thermal ink jet printing systems have a printhead comprising one or more ink filled channels that communicate with a relatively small ink supply chamber at one end and have an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzle a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse representative of data signals, to momentarily vaporize the ink and form a bubble which expels an ink droplet. The ink droplets expelled from each nozzle by the growth of the bubbles which cause a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and the velocity of the droplet which, after separation from the nozzle, travels in a substantially straight line towards a recording medium, such as paper.
One preferred method of fabricating thermal ink jet printheads is to form the heating elements on the surfaces of one silicon wafer and the channels and small ink supply chamber or reservoir in the surface of another silicon wafer. The two wafers are precisely aligned to insure that the heating elements are aligned to their corresponding channels and then the two wafers are bonded together. The individual printheads are obtained by dicing the two bonded wafers. This general process has been described in U.S. patent application Ser. No. 719,410 to Hawkins et al, filed Apr. 3, 1985, now U.S. Pat. No. 4,601,777. A critical part of this assembly process is the bonding adhesive and its application. Since two silicon wafers are mated that are extremely flat, a thin adhesive coat is sufficient to bond the two together and a much thicker coat will clog the channels. In earlier printhead fabrication processes, adhesive was spray coated on the entire surface of the wafer containing the ink reservoirs and then the channels were later diced into the wafer with a precise dicing saw. In this manner, the ink channels were clear of adhesive although an adhesive film was left inside each of the reservoirs. This was less than optimal because adhesive coating in the reservoir could break loose and clog channels. Nevertheless, this system of applying adhesive worked whenever the channels were diced after the adhesive was applied.
In the fabricating process disclosed in the above-mentioned patent application to Hawkins et al, the ink channels are fabricated by anisotropic etching of the silicon and are created simultaneously with the reservoir. This method offered a number of significant advantages, but one problem encountered was that the fluid structures (namely, reservoir, fill hole, and channels) are simultaneously created before the adhesive application step. This meant that the former method of applying adhesive over the entire wafer is no longer practical since the adhesive coats the inside of the channels as well. In fact, adhesive tends to flow to the apex of each channel. The result was that ink channels that have an adhesive coat not only presented a potential that the adhesive would break off and clog the channel during operation, but the adhesive changes the effective dimension or cross-sectional area of the channel and therefore the nozzle, so that the tight dimensional control afforded by the anisotropic etching of the channels is lost.
The critical problem then was how to adhesively bond the surface of the silicon wafer containing the sets of heating elements to the surface of the other silicon wafer containing the plurality of small ink reservoirs and associated sets of ink channels with the adhesive being applied to only the mating interfaces between the two wafers. Such a bonding process would mean that all of the fluid structures, that is, the fill hole, ink reservoir, and channels, would be clear of adhesive. A review of the prior art discussed below offered no help.
U.S. Pat. No. 4,284,457 to Stonier et al discloses use of a fabric coated on one side with an adhesive which in turn is covered by a release layer. This sheet of adhesive coated fabric is laid on the edge of a honeycomb and partially cured by heat and pressure. The release sheet prevents the adhesive from contacting the plates through which the pressure is applied. The adhesive is allowed to harden and become brittle so that it is frangible. The fabric is then removed, taking with it most of the adhesive. The adhesive left is that portion adhering to the honeycomb edges that is broken free from the other part of the hardened adhesive layer in the fabric.
U.S. Pat. No. 4,147,579 to Schade discloses a layer of adhesive placed on a conductive substrate. The layer of adhesive must be electrically insulative and permit shapes or parts to be stamped therefrom without separating the special insulating adhesive from the substrate. Electrical components are subsequently adhered to the insulative adhesive which does not flow or run under heat or pressure.
U.S. Pat. No. 4,465,538 to Schmoock discloses placing adhesive on a substrate in a circuit-like pattern and moving a foil layer on a carrier web into contact with the substrate surface having the adhesive patterned thereon. Adhesive is cured and the foil stripped away. The adhesive keeps that portion of the foil contacting it and allows the remainder of the foil to stay with the carrier web. Thus, the foil in the adhesive pattern remains on the substrate for electroplating of the foil to the proper thickness. This technique is used to economically produce circuit boards.
U.S. Pat. No. 3,089,800 to Colfer et al discloses an applique that comprises a metal foil having a predetermined configuration, a release sheet, and adhesive coating therebetween, the adhesive coating being less tenacious to the release sheet than to the foil whereby substantially all of the adhesive remains on the foil when the release sheet is removed. Therefore, this patent is concerned with an adhesive which facilitates total release from the release sheet, a feature unacceptable for the present invention. This is because the surface energy of a low viscosity adhesive would be such that, in a thickness range of a few microns or less, the adhesive would tend to bead up on a release sheet, such as Teflon.RTM., thus degrading adhesive uniformity.